1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In fixing control of an image forming apparatus, typically, a halogen lamp heater is used as a fixing heater, and temperature of the fixing heater is controlled by ON/OFF control. In order to control the temperature more finely, there is a control method of supplying drive voltage of appropriately selected half-waves of AC waveform to a halogen lamp heater.
In this kind of control method, the number of half-waves of the AC waveform in a predetermined period (duty cycle) is appropriately selected according to the heat amount to be required, and the effective value of drive voltage to be supplied to a halogen lamp heater changes according to the selected number of half-waves of the AC waveform.
Meanwhile, for a halogen lamp heater, there is a reference voltage with which halogen cycle occurs most efficiently. If the number of selected half-waves (i.e., the selected number of half-waves) of the AC waveform is small, and hence the effective value of drive voltage to be supplied to a halogen lamp heater is smaller than the reference voltage, the temperature of a filament (tungsten) of the halogen lamp heater becomes low, and a phenomenon that the filament is eaten away, called “chemical attack”, occurs.
Then, there is a heater control device (Japanese Patent Application Publication No. 2011-257604) which, in order to let halogen cycle occur in the standby mode too, fully turns on (ON) a halogen lamp heater at predetermined time intervals, and when a filament thereof reaches a predetermined temperature, supplies drive voltage of appropriately selected half-waves of the AC waveform to the halogen lamp heater without turning off (OFF) the halogen lamp heater, thereby reducing flicker as well as preventing breaking of the filament.
The heat amount required for image forming depends on the type and/or the thickness of paper, which is a recording medium. For example, in the case of image forming on thin paper, the required heat amount is small, and therefore the application pattern for drive voltage has a small number of half-waves of the AC waveform selected in a predetermined period (low duty cycle). In this case, the duty cycle of the application pattern is limited to a predetermined value or more, which can prevent chemical attack and extend life of the halogen lamp heater.
However, even if, in order to prevent chemical attack, the duty cycle of the application pattern is limited to the predetermined value or more in the case of image forming on thin paper, the heater cannot be kept ON if the heat amount generated by turning on the heater with the application pattern having the duty cycle is more than the required heat amount, and needs to be turned off at appropriate timing to make the heat amount close to the required heat amount. Such temperature control cannot stabilize the temperature of a fixing roller.
Then, a plurality of heaters of the same light distribution, the sum of the heat amounts of the heaters being the maximum heat amount required for “fixing” (i.e., fixing the toners to the paper), is provided, and the heaters are used in combination, and turned on with the application pattern having the duty cycle of the predetermined value required to prevent chemical attack or more, thereby being controlled to continuously generate a predetermined heat amount. Further, the heaters include at least one heater which generates a heat amount being the minimum heat amount required for fixing or less when turned on with the application pattern having the duty cycle of the predetermined value. This can stabilize the temperature of a fixing roller.
However, in order to continuously generate a predetermined heat amount by combination of the heaters, it is necessary to change the lighting with the heaters at appropriate timing. If the heat amount required for fixing is a heat amount near the borderline where switching from one heater to another is performed, frequent switching between these two heaters may occur, which causes flicker.